DNA Structure: Puzzle Number 1

In their STM study of nucleic acids (1), Lee et al. recorded in Figure 3(b)
lengthy stretches of at least four well-resolved linear DNA duplexes.
The vital feature of this image of DNA molecules is that no plectonaemic
winding can be discerned. That is, they show no sign of being
double-helical. The duplexes consist of long, paired, helices with a
consistent, uninterrupted parallel (or antiparallel) contour running between
the two strands of each of the four duplexes, whose highlighted features
remain exactly in phase with each other over their whole lengths, extending
to some 17 - 20 full helical turns.
1 Scanning Tunnelling Microscopy of Nucleic Acids; G. Lee, P.G.
Arscott, V.A. Bloomfield & D. Fennell Evans; SCIENCE Vol 244 (1989) 475 -
477
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Clive Delmonte